Back in the SearchProviderList array, look in each Dictionary (besides 4, which is the DuckDuckGo one you just made) to see which one has a boolean field called 'Default'. Delete the variable entirely (but leave the one in the DuckDuckGo dictionary). Ed. Note: I didn't find another Dictionary with the Default value, so I skipped this step.

Exit iFile and launch System Preferences. Go to Safari > Search Engine and select the new DuckDuckGo option you just created.

Close Safari in the multitasking bar (if applicable), then relaunch it and play with your new search engine!

It's much easier to do than the daunting wall of text above implies, and completely worth the outcome. Obviously, if you want to use a different search engine, you can do that too—you'll just have to find values for the Search and Suggest URLs. And, as always, be careful when messing with system files and do so at your own risk, although the above did work perfectly for me.

Ed. Note: The above trick worked perfectly for me too. I tried this with iExplorer on the desktop and it didn't work, though, so it looks like you'll have to go through iFile on your device. It'll take a bit longer, but it works like a charm.